Transitioning from MIMO to SISO to save power

ABSTRACT

Various example embodiments are disclosed. According to an example embodiment, an apparatus may include at least one processor and at least one memory. The at least one memory may include computer-executable code that, when executed by the processor, is configured to cause the apparatus to send a message to a node in wireless communication with the apparatus, the message indicating a transition by the apparatus from multiple-input multiple-output (MIMO) to single-input single-output (SISO), and transition from wireless MIMO communication with the node to wireless SISO communication with the node after sending the message to the node.

PRIORITY CLAIM

This Application claims the benefit of priority based on U.S. Provisional Application No. 61/406,319, filed on Oct. 25, 2010, entitled, “Transitioning From MIMO To SISO To Save Power,” the disclosure of which is hereby incorporated by reference.

TECHNICAL FIELD

This description relates to wireless communication devices.

BACKGROUND

Wireless communication devices may be disconnected from external power sources. As such, their available power, typically supplied by a battery, may be limited.

SUMMARY

The details of one or more implementations are set forth in the accompanying drawings and the description below. Other features will be apparent from the description and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram showing two nodes in wireless communication according to an example embodiment.

FIG. 2 is a vertical time sequence diagram showing communication between the two nodes according to an example embodiment.

FIG. 3 is a diagram of a management frame according to an example embodiment.

FIG. 4 is a diagram of a data frame according to an example embodiment.

FIG. 5 is a block diagram of an apparatus according to an example embodiment.

DETAILED DESCRIPTION

FIG. 1 is a diagram showing two nodes 102, 104 in wireless (or unguided) communication according to an example embodiment. The nodes 102, 104 may include, for example, IEEE 802.11 wireless local area network (WLAN) nodes (such as a station 102 and an access point 104), IEEE 802.16 Worldwide inter-Operability for Microwave Access (WiMAX) nodes, IEEE 802.15 Bluetooth nodes, or cellular telephones/smartphones and base stations, according to various example embodiments. While the terminology of IEEE 802.11 is used herein, the techniques and methods described herein are applicable to a wide range of wireless (or unguided) communication technologies.

Station 102 may include a station or apparatus in wireless communication with an access point 104. The station 102 may disconnected from any external power source and rely on battery power. The station 102 may have limited battery power, and it may be desirable to reduce the use of the battery power. At times, it may desirable for the station 102 to perform techniques to reduce the use of battery power.

The nodes 102, 104 may be capable of communication using both multiple input, multiple output (MIMO) techniques, in which the transmitting node 102, 104 has two or more transmitters, and sends two or more unique data streams or signals in the same frequency, and the receiving node combines the multiple streams or signals to recover the original transmitted streams, as well as single input, single output (SISO) techniques, in which only a single data stream or signal is transmitted. MIMO may be advantageous by sending or transmitting multiple outputs, creating spatial multiplexing to allow for transmission and reception of a better signal. Similarly, MIMO may receive at multiple inputs or antennas, maximizing the possibility of receiving a strong signal. However, MIMO may consume more power than SISO at both the transmitting node by transmitting two or more streams or signals, and at the receiving node by processing the two or more streams or signals; at times, it may be helpful to transition to SISO to conserve power.

The access point 102 may transition from MIMO to SISO to save power under various conditions. For example, the station 102 may transition to an IEEE 802.11 power save mode, and transition from MIMO to SISO concurrently with transitioning to the power save mode. The station 102 may also transition from MIMO to SISO when entering a power save mode using unscheduled automatic power save delivery (U-APSD). The station 102 may also selectively use MIMO for certain types of traffic, such as voice and video traffic, while using SISO for other types of traffic, such as best effort and background traffic. The station 102 may also transition from MIMO to SISO based on remaining battery power, and enter the MIMO communication when battery power is low, or has reached or passed a predetermined threshold. The station 102 may also communicate in MIMO or SISO based on a transmission queue depth; when the transmission queue depth is high, indicating that there are many packets that need to be sent out or transmitted, the station 102 may communicate in MIMO to ensure a high effective data rate, whereas when the transmission depth is low, the station 102 may transition to, or communicate in, SISO, to reduce power consumption.

FIG. 2 is a vertical time sequence diagram showing communication between the nodes 102, 104 according to an example embodiment. The diagram shows communication between the station 102 and the access point 104.

The station 102 and the access point 104 may engage in association procedures. The station 102 and access point 104 may have engaged in probe request and probe response communications to negotiate security parameters, as well as authentication request and authentication response communications to negotiate authentication parameters and/or authenticate the station 102. The station 102 may send an association request 202 to the access point 104. The association request 202 may include a management frame such as the management frame 300 shown in FIG. 3. The association request 202 and/or management frame 300 may include, for example, a medium access control (MAC) header 302, a frame body 304, and a frame check sequence (FCS) 306. The MAC header 302 may include a frame control field 308, a duration field 310, a destination address (DA) field 312, a source address (SA) field 314, a basic service set identification (BSSID) field 316, a sequence control field 318, and a high throughput (HT) control field 320.

The frame body 304 of the association request 202 may include, for example, a capability subfield, a listen interval subfield, a service set identifier (SSID) subfield, a supported rates subfield, an extended supported rates subfield, a power capability subfield, a supported channels subfield, a robust security network (RSN) subfield, a quality of service (QoS) capability subfield, an HT capabilities subfield, a 20/40 basic service set (BSS) subfield, an extended capabilities subfield, and any vendor-specific information elements. In an example embodiment in which the station 102 uses MIMO only for certain types of traffic, such as voice and video traffic (discussed below), the frame body 304 of the association request 202 may also include a subfield and/or element ID indicating which types of traffic will be sent using MIMO and/or which types will be sent using SISO. The subfield and/or element ID indicating which types of traffic will be sent using MIMO may identify only the traffic types which will be sent using MIMO, leaving the SISO traffic types to be inferred by their lack of identification, may identify only the traffic types which will be sent using SISO, leaving the MIMO traffic types to be inferred by their lack of identification, may identify a priority level above or below which traffic will be sent using either MIMO or SISO, or may specifically identify which traffic types will be sent using each of MIMO and SISO, according to various example embodiments.

The association request 202 may indicate a request by the station 102 to associate with a wireless network served by the access point 104. The association request 202 may also indicate the station 102's MIMO and/or SISO communication capabilities (which may, for example, be included in the HT capabilities subfield of the frame body 304).

The access point 104 may respond to the association request 202 by sending an association response 204. The association response 204 may also include a management frame such as the management frame 300 shown in FIG. 3. The association response 204 and/or management frame 300 may include, for example, the medium access control (MAC) header 302, the frame body 304, and the frame check sequence (FCS) 306. The MAC header 302 may include the frame control field 308, the duration field 310, the destination address (DA) field 312, the source address (SA) field 314, the basic service set identification (BSSID) field 316, the sequence control field 318, and the high throughput (HT) control field 320.

The frame body 304 of the association response 204 may include, for example, a capability subfield, a status code subfield, an association identifier (AID) subfield, a supported rates subfield, an extended supported rates subfield, an enhanced distributed channel access (EDCA) parameter set subfield, an HT capabilities subfield, an HT operation subfield, a 20/40 BSS coexistence subfield, an overlapping BSS scan parameters, subfield, an extended capabilities subfield, and any vendor-specific information elements or subfields.

The association response 204 may grant the request by the station 102 to associate with and/or enter the wireless network served by the access point 104. The association response 204 may also indicate that the access point 104 is capable of communicating in both MIMO and SISO modes (which may, for example, be included in the HT capabilities subfield of the frame body 304). The access point 104 may also advertise or indicate whether it is capable of MIMO communication in beacon frames and/or the probe response frame(s).

After the association has been completed by the exchange of the association request 202 and the association response 204, the station 102 and the access point 104 may engage in MIMO communication 206. The station 102 and access point 104 may engage in MIMO communication 206 by sending or transmitting signals, which may include data and/or control information, via multiple outputs or antennas, and/or by receiving signals via multiple inputs or antennas. During the MIMO communication, the access point 104 may indicate its capability of supporting a low power mode and/or unscheduled automatic power save delivery (U-APSD). These capabilities may be indicated, for example, in periodic beacon frames sent or transmitted by the access point 104.

While this MIMO communication is ongoing, the station 102 may experience a transition trigger 208. The transition trigger 208 may include any trigger or determination to enter the SISO mode. The transition trigger 208 may include, for example, a determination to enter a power save mode, a determination that the battery power of the battery in the station 102 is low, or a determination that a transmission queue depth of the station 102 is sufficiently low to enter SISO communication.

Upon the transition trigger, the station 102 may send a transition message 210 to the access point 104. The transition message 210 may indicate that the station 102 will transition from the MIMO communication to the SISO communication. In an example embodiment, the indication of the transition may be included in an action frame which may also indicate that the station 102 is entering a low-power state or U-APSD. The transition message 210 may include an action frame. The transition message 210 may include a NULL or QNULL frame, such as a data frame with a subtype set to Null (no data) or QoS Null (no data).

FIG. 4 is a diagram showing a data frame 400 according to an example embodiment. The transition message 210 sent by the station 102 to the access point 104 to indicate the transition from MIMO communication to SISO communication may include the fields shown in the data frame 400 of FIG. 4. The data frame 400 and/or transition message 210 may include a MAC header 402 and a frame check sequence 404. The MAC header 402 may include a frame control field 406, a duration field 408, an address 1 field 410, an address 2 field 412, an address 3 field 414, a sequence control field 416, an address 4 field 418, and a QoS control field 420. The address 1 field 410 may include the address of the intended recipient(s) of the data frame 400 or transition message 210, such as the access point 204, and the address 2 field 412 may include the address of the station 102 that is transmitting the data frame 400 of transition message 210. The QoS control field 420 may indicate that the data frame 400 and/or transition message 210 is a transition message and that the access point 102 is transitioning from MIMO to SISO, such as by including a subtype value corresponding to Null (no data) or QoS Null (no data), according to example embodiments.

The access point 104 may receive the transition message 210 and send an acknowledgement message, such as a transition acknowledgement 212 to the station 102. The transition acknowledgement 212 may have a similar format to the transition message 210, such as by including the fields of the data frame 400. The QoS control field 420 of the transition acknowledgment 212 may indicate that the data frame 400 and/or transition acknowledgment 412 is a transition acknowledgment, such as by including a subtype value corresponding to CF-Ack (no data), CF-Ack+CF-Poll (no data), QoS CF-Poll (no data), or QoS CF-Ack+CF-Poll (no data), according to example embodiments.

The transition acknowledgment 212 and/or data frame 400 sent by the access point 104 to the station 102 may indicate that the access point 104 has received and accurately processed the transition message 210, and is prepared for the station 102 to transition into SISO and commence communication in SISO. Upon receipt of the transition acknowledgement 212 from the access point 104, the station 102 may engage in SISO communication 214 with the access point 104. In SISO communication 214, the station 102, and/or the access point 104, may communicate by sending data using only a single output or antenna to send messages, and also using only a single input or antenna to receive messages. The station 102 and the access point 104 may engage in SISO communication 214 until the station 102 determines to return to MIMO communication 216. The station 102 may determine to return to MIMO communication 216 based on events which mirror the events which trigger the transition into SISO, such as battery power being high enough or transition queue depth being high enough.

In the example of transitioning to SISO communication 214 based on entering power save mode, the station 102 may engage in SISO communication while in the power save mode. In the power save mode, the station 102 may sleep during most of the time that the access point 104 is communicating with other nodes, but may wake up, such as by powering on the receiver, during expected reception times (which may be indicated by a beacon transmitted by the access point 104), to determine whether data addressed for the station 102 is scheduled to be sent by the access point 104 to the station 102. The access point 104 may buffer packets destined for the station 102 during the station's 102 sleep mode, and may send the buffered packets to the station 102 when the station 102 periodically wakes. If the beacon frame does indicate that data is scheduled to be sent from the access point 104 to the station 102, the station 102 may stay awake and wait for the data or frame, or may wake up at a time indicated by the beacon to receive and process the data or frame. If the beacon does not indicate that the access point 104 will be sending data to the station 102, the station 102 may remain asleep until the next beacon frame or scheduled reception time.

In the SISO communication 214 during U-APSD, the station 102 may spend time in a dozing state which consumes less power. The station 102 may doze until specific times when the access point 104 is scheduled to send frames or packets to the station 102. The station 102 may also request the access point 104 to send buffered frames by sending a trigger frame indicating the request to send the buffered frames; the trigger frame may be included in an action frame, according to an example embodiment. During the times when the access point 104 is scheduled to send frames or packets to the station 102, the station 102 may awake to receive the frames or packets. If the station 102 is entering the SISO communication or transitioning from MIMO communication to SISO communication based on remaining battery power, the station 102 may continuously or periodically monitor the battery power. The station 102 may compare the measured battery power to a threshold; when the battery power is at or below the threshold, the station 102 may determine to transition from MIMO communication to SISO communication. Upon making the determination based on the comparison of the measured battery power to the threshold level, the station 102 may send the transition message 210 to the access point 104, and upon receipt of the transition acknowledgement 212 transition from the access point 104, cease or transition from MIMO communication and commence or transition to the SISO communication.

In the example of transitioning based on the transmission queue depth, the station 102 may monitor and/or compare a transmission queue depth to a threshold. The transmission queue depth may be based on a number of packets or frames ready to be sent by the station 102 to the access point 104. The station 102 may buffer frames or packets in a memory, and be ready to send the packets or frames to the access point 104 during times that the access point 104 schedules for the station 102 to send data to the access point 104. When the transmission queue depth has met or gone below the transmission queue depth threshold, the station 102 may determine to transition from MIMO communication to SISO communication. Upon making the determination to transition from MIMO communication to SISO communication, the station 102 may send the transition message 210 to the access point 104. Upon receiving the transition acknowledgement 212 from the access point 104, the station 102 may transition from MIMO communication to SISO communication and commence SISO communication 214 with the access point 104.

In the example in which MIMO mode is used for only certain types of traffic, the station 102 may not need to send the transition message 210 or receive to the access point 104 or receive the transition acknowledgement 212 from the access point 104. In this example, the station 102 and access point 104 may have negotiated the parameters for MIMO and SISO communication during the association. For example, the association request message 202 may have previously indicated for which types of traffic the station 102 will communicate using MIMO and for which the station 102 will communicate using SISO. For example, the association request 202 (which may have the format of the management frame 300) may indicate for which traffic types the station 102 will use MIMO for which traffic types the station 102 will use SISO. The indication of which traffic types will use MIMO and which traffic types will use SISO may be included in a subfield and/or element ID of the frame body 304 of the association request 202. For example, the subfield and/or element ID of the frame body 304 may indicate that the station 102 may communicate with the access point 104 using MIMO for voice traffic and video traffic but will use SISO for best effort traffic and background traffic. Thus, the station 102 and access point 104 may already understand during the MIMO communication and/or SISO communication that the station 102 will be communicating in the indicated type of communication protocol for the indicated type of traffic. The station 102 and access point 104 may not need to engage exclusively in MIMO communication 206 or SISO communication 214, but may engage in blended communication techniques with MIMO communication being used for higher priority traffic types such as voice and video traffic, and SISO communication for lower priority traffic such as best effort and background traffic. In an example embodiment, the station 102 may dynamically change which traffic types use MIMO communication and which traffic types use SISO communication; the station 102 may, for example, send an action frame or management frame to the access point 104 indicating which types of traffic will use MIMO and/or which types of traffic will use SISO.

FIG. 5 is a block diagram of an apparatus 500 according to an example embodiment. The apparatus 500 may include a wireless node such as the station 102, and may include an IEEE 802.11 node, IEEE 802.16 nodes, IEEE 802.15 node, or cellular telephone/smartphone, netbook, tablet PC, iPad, or laptop computer, as non-limiting examples. The apparatus 500 may include a controller 502 and memory 504. The memory 504 may store data and instructions. The controller 502 may execute the instructions stored in the memory 504 to perform any of the tasks, methods, or functions described herein. The apparatus 500 may also include a rechargeable or non-rechargeable battery 506. The controller 502 may monitor the power level of the battery 506 to determine whether to transition to from the MIMO communication to the SISO communication, as described above.

The apparatus 500 may also include a baseband processor 508. The baseband processor may convert between data generated or processed by the controller 502 and baseband signals received or transmitted by a transceiver 510 according to a communication standard, such as IEEE 802.11, IEEE 802.16, IEEE 802.15, or any cellular communication technology. The apparatus 500 may include the transceiver 500, which may convert between the baseband signals and radio frequency (RF) signals, and serve as the physical interface for communicating with other wireless devices, such as the access point 104.

Implementations of the various techniques described herein may be implemented in digital electronic circuitry, or in computer hardware, firmware, software, or in combinations of them. Implementations may implemented as a computer program product, i.e., a computer program tangibly embodied in an information carrier, e.g., in a machine-readable storage device, for execution by, or to control the operation of, data processing apparatus, e.g., a programmable processor, a computer, or multiple computers. A computer program to execute the tasks, methods, or functions described above, can be written in any form of programming language, including compiled or interpreted languages, and can be deployed in any form, including as a stand-alone program or as a module, component, subroutine, or other unit suitable for use in a computing environment. A computer program can be deployed to be executed on one computer or on multiple computers at one site or distributed across multiple sites and interconnected by a communication network.

Method steps may be performed by one or more programmable processors executing a computer program to perform functions by operating on input data and generating output. Method steps also may be performed by, and an apparatus may be implemented as, special purpose logic circuitry, e.g., an FPGA (field programmable gate array) or an ASIC (application-specific integrated circuit).

Processors (or controllers) suitable for the execution of a computer program include, by way of example, both general and special purpose microprocessors, and any one or more processors of any kind of digital computer. Generally, a processor will receive instructions and data from a non-transitory, computer-readable read-only memory or a non-transitory, computer-readable random access memory, or both. Elements of a computer may include at least one processor for executing instructions and one or more memory devices for storing instructions and data. Generally, a computer also may include, or be operatively coupled to receive data from or transfer data to, or both, one or more mass storage devices for storing data, e.g., magnetic, magneto-optical disks, or optical disks. Information carriers suitable for embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, e.g., EPROM, EEPROM, and flash memory devices; magnetic disks, e.g., internal hard disks or removable disks; magneto-optical disks; and CD-ROM and DVD-ROM disks. The processor and the memory may be supplemented by, or incorporated in special purpose logic circuitry.

To provide for interaction with a user, implementations may be implemented on a computer having a display device, e.g., a cathode ray tube (CRT) or liquid crystal display (LCD) monitor, for displaying information to the user and a keyboard and a pointing device, e.g., a mouse or a trackball, by which the user can provide input to the computer. Other kinds of devices can be used to provide for interaction with a user as well; for example, feedback provided to the user can be any form of sensory feedback, e.g., visual feedback, auditory feedback, or tactile feedback; and input from the user can be received in any form, including acoustic, speech, or tactile input.

Implementations may be implemented in a computing system that includes a back-end component, e.g., as a data server, or that includes a middleware component, e.g., an application server, or that includes a front-end component, e.g., a client computer having a graphical user interface or a Web browser through which a user can interact with an implementation, or any combination of such back-end, middleware, or front-end components. Components may be interconnected by any form or medium of digital data communication, e.g., a communication network. Examples of communication networks include a local area network (LAN) and a wide area network (WAN), e.g., the Internet.

While certain features of the described implementations have been illustrated as described herein, many modifications, substitutions, changes and equivalents will now occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the embodiments of the invention. 

What is claimed is:
 1. An apparatus comprising: at least one processor; and at least one memory comprising computer-executable code that, when executed by the at least one processor, is configured to cause the apparatus to: determine to transition from multiple-input multiple-output (MIMO) communication to single-input single-output (SISO) communication based on a depth of a transmission queue, the transmission queue storing data to be sent from the apparatus to a node in wireless communication with the apparatus; send a message to the node based on the determining, the message indicating a transition by the apparatus from MIMO communication to SISO communication; and transition from MIMO communication with the node to SISO communication with the node after sending the message to the node.
 2. The apparatus of claim 1, wherein the computer-executable code is configured to cause the apparatus to transition from the MIMO communication with the node to the SISO communication with the node after receiving an acknowledgment from the node acknowledging the message indicating the transition.
 3. The apparatus of claim 1, wherein the computer-executable code is further configured to cause the apparatus to: determine to enter a power save mode; and determine to transition from the MIMO communication to the SISO communication based on determining to enter the power save mode and the depth of the transmission queue, wherein the apparatus is configured to send the message and transition from the MIMO communication to the SISO communication based on the determining to transition from the MIMO to the SISO.
 4. The apparatus of claim 1, wherein the computer-executable code is further configured to cause the apparatus to: determine to transition from the MIMO communication to the SISO communication based on a measured level of battery power in the apparatus and the depth of the transmission queue, wherein the apparatus is configured to send the message and transition from the MIMO communication to the SISO communication based on the determining.
 5. The apparatus of claim 1, wherein the message comprises a frame indicating that the apparatus is entering a power save mode and the transition by the apparatus from MIMO communication to SISO communication.
 6. The apparatus of claim 1, wherein the computer-executable code is further configured to cause the apparatus to associate with the node, the associating including receiving a message from the node indicating that the node supports MIMO communication.
 7. The apparatus of claim 1, wherein the computer-executable code is further configured to cause the apparatus to: send a message to the node indicating a transition by the apparatus back from SISO communication to MIMO communication; and transition back from SISO communication to MIMO communication with the node after sending the message indicating the transition back from SISO communication to MIMO communication.
 8. The apparatus of claim 1, wherein the node in wireless communication with the apparatus includes an IEEE 802.11 access point.
 9. An apparatus comprising: at least one processor; and at least one memory comprising computer-executable code that, when executed by the at least one processor, is configured to cause the apparatus to: send a frame to a node, the node indicating at least a first traffic type for which the apparatus will wirelessly communicate according to a multiple-input multiple-output (MIMO) communication protocol and at least a second traffic type for which the apparatus will wirelessly communicate with the node according to a single-input single-output (SISO) communication protocol; wirelessly communicate with the node according to the MIMO communication protocol for at least the first traffic type; and wirelessly communicate with the node according to the SISO communication protocol for at least the second traffic type.
 10. The apparatus of claim 9, wherein the computer-executable code is further configured to cause the apparatus to: determine to transition from the MIMO communication protocol to the SISO communication protocol based on a changing priority level of traffic sent between the apparatus and the node, wherein the apparatus is configured to transition from the MIMO communication protocol to the SISO communication protocol based on the determining.
 11. The apparatus of claim 9 wherein the computer-executable code is configured to cause the apparatus to: communicate with the node according to the MIMO communication protocol for voice and video traffic; and communicate with the node according to the SISO communication protocol for best effort and background traffic.
 12. The apparatus of claim 9, wherein the frame includes an association request frame.
 13. The apparatus of claim 9, wherein the frame includes an updated MIMO frame.
 14. The apparatus of claim 9, wherein the node includes an IEEE 802.11 access point.
 15. A non-transitory, computer-readable medium comprising computer-executable code stored thereon that, when executed by a data processing apparatus, is configured to cause the data processing apparatus to: determine to transition from multiple-input multiple-output (MIMO) communication to single-input single-output (SISO) communication based on a depth of a transmission queue, the transmission queue storing data to be sent from the apparatus to a node in wireless communication with the apparatus; send a message to the node based on the determining, the message indicating a transition by the apparatus from MIMO communication to SISO communication; and transition from MIMO communication with the node to SISO communication with the node after sending the message to the node.
 16. The computer-readable medium of claim 15, wherein the computer-executable code is configured to cause the apparatus to transition from the MIMO communication with the node to the SISO communication with the node after receiving an acknowledgment from the node acknowledging the message indicating the transition.
 17. The computer-readable medium of claim 15, wherein the computer-executable code is configured to cause the data processing apparatus to: determine to enter a power save mode; determine to transition from the MIMO communication to the SISO communication based on determining to enter the power save mode and the depth of the transmission queue; and send the message and transition from the MIMO communication to the SISO communication based on the determining to transition from the MIMO to the SISO.
 18. The computer-readable medium of claim 15, wherein the computer-executable code is further configured to cause the data processing apparatus to: determine to transition from the MIMO communication to the SISO communication based on a measured level of battery power in the apparatus and the depth of the transmission queue, wherein the sending the message and transitioning from the MIMO communication to the SISO communication is based on the determining.
 19. The computer-readable medium of claim 15, wherein the message comprises a null frame indicating that the apparatus is entering a power save mode and the transition by the data processing apparatus from MIMO communication to communication.
 20. The computer-readable medium of claim 15, wherein the computer-executable code is further configured to cause the data processing apparatus to associate with the node, the associating including receiving a message from the node indicating that the node supports MIMO communication. 